1. Field of the Invention
The present invention relates generally to a dynamic load alleviation system and, more particularly, to the design, manufacture, and operation of an active control surface modal device.
2. Description of the Prior Art
Civil and military aircraft encounter a number of dynamic load conditions resulting from fluctuating air loads. Such environments lead to ride discomfort, structural fatigue damage and degradation of flight performance. Hence, there is a need to improve the fatigue life and flight performance of civil and military aircraft.
Buffet is a turbulence phenomenon that originates from flow separated wake created behind any aerodynamic lifting surfaces or bodies. In a typical example illustrated in FIG. 1, a modern high performance aircraft 20 incorporates a strake 22 on a fuselage 24 extending into a leading edge 26 of a wing 28. In a typical fashion, the release of strong vortices 30 behind the leading edge extension, or strake, of the aircraft are depicted. At some distance from its origin, as depicted at 32, the vortex core bursts and engulfs the tail 34. In this manner, micro vortices are generated which cause a fluctuating pressure field of random nature and severely excite the tail of the aircraft.
Such dynamic environments have caused fatigue failures at the root and the mid-section of the vertical tail or tails of aircraft on which they appear. Tail skin fasteners around the rudder hinge line often disappear. Buffet induced excitation can also lead to dynamic problems of engine mounts that are dose to the tail root section. Consequently, these problems increase the life cycle cost of an aircraft. As a result, the Government procuring agencies and aircraft manufacturers are interested in finding solutions to these problems.
Buffet load problems have been extensively studied by a number of its investigators. These studies were actually conducted in two parts. The first part of these studies was devoted to the understanding of the physical characteristics of the fluctuating pressures, while the second part was focused on the remedial procedures. The remedial procedures offer passive and active control methods. A brief account of these studies will now be discussed.
The passive methods include the design of various configurations of the leading edge extension (LEX) with and without fences on the LEX. The fences serve to break up the vortex core and consequently reduce the vortex strength. On the other hand, strong vortex cores are required to generate suction pressure to achieve super maneuver performance of high performance aircraft. Although fences reduce the root bending moment on the vertical tail, these were not recommended for the production series aircraft for two reasons: (1) they are expensive to install, and (2) they degrade the quality of flow intended for high angle of attack maneuvers. One study reports an alternate passive method that uses blowing and suction of air around LEX to suppress buffeting.
Two different principles are used in active buffet load control technology.
Principle No. 1, so-called, uses aerodynamic effectors (control surfaces or active control surface modes generally of the presented below in this disclosure) to generate aerodynamic damping that reduces buffet induced oscillations. The deployment of these effectors is achieved by means of actuators, either of conventional hydraulic actuators having low frequency bandwidth or smart actuators having broad band frequency range. The power requirement is directly related to the amount of damping required, or in other words, deployment amplitude of the effectors. This is a positively robust approach.
Principle No. 2, so-called, uses an anti-wave generation method in which the structure is excited at its natural frequencies and out-of-phase with the forcing signals. In this approach, cancellation can be achieved only at discrete frequencies of the structural modes. At other frequencies, enormous power is required to excite the structure to generate aerodynamic damping. Since buffet is a broad band phenomenon, it can force all structural modes at the same time. The wave cancellation method can be effective only at one frequency at a time, which is the principle behind surface mounted piezoactuators.
One investigator and his associates employ the first principle to reduce buffet induced structural stresses. They activate the rudder using conventional hydraulic or pneumatic actuators to generate out-of-phase unsteady aerodynamic loads to suppress vibration of the tail. Unfortunately, there are two main problems in this approach. The first problem is that the flight control system and the buffet control system use the same control surface, which reduces the availability of the control surface for either purpose. Also, interference with the flight control is an undesirable aspect that pilots do not like. In addition, the conventional actuators are limited in the frequency bandwidth which makes it difficult to swing the massive rudder at higher frequencies about the hinge line. For example, the rudder of an F/A-18 weighs about 64 pounds. Hence, the ability of such a typical airfoil and its actuators to function in a wide-band buffet spectrum is significantly limited. One wind tunnel study sponsored by NASA, reported a 60% reduction of bending moment in a buffet load environment. This particular wind tunnel study employed a ⅙-scale F-18 model with actively controlled surface mounted piezoelectric actuators on the vertical fins. The disadvantage of this approach will be discussed shortly.
While surface mounted piezoelectric actuators proposed for some of these programs are good candidates for wind tunnel models they have no practical value for production scale aircraft for the following reasons:
surface mounted piezoelectric actuators cannot produce anti-mode waves to counteract the buffet excitation and cannot provide large surface strains;
the model studies assumed that actively controlled piezoelectric actuators provide necessary mechanical damping to suppress vibration. This assumption does not hold well for full-scale aircraft. In reality, aerodynamic damping plays a greater role than the mechanical damping. The merits of aerodynamic damping have been demonstrated in active flutter suppression technologies.
surface mounted actuators and electrical contacts may fail due to fatigue and erosion;
surface mounted actuators cause flow separation;
there is a weight penalty if the surface mounted actuators are used in large quantities; and
high voltage input may be required which would cause the risk of arcing across structural joints.
Gust environment is another important aspect of aircraft dynamic loads that arise from atmospheric turbulence. Here, aircraft penetrate a sinusoidal gust wave resulting in loss or gain of vertical lift force. Thus, in this instance, the aircraft is subjected to external excitations causing ride discomfort and structural fatigue damage.
Still another design criterion that requires a careful consideration is flutter, or aeroelastic instability. Flutter is a self-excited oscillatory phenomenon that results in structural instability leading to catastrophic destruction when the flight speed exceeds the design speed limit.
A small number of patents are typical of the known prior art attempting to reduce dynamical loads. For example, U.S. Pat. No. 4,706,902 to Destuynder et al. discloses an active control method of reducing the buffet loads. A device is used to detect buffet onset and active control system is used to actuate a number of control surfaces about their hinge lines to generate aerodynamic damping forces to oppose buffet excitation. Since buffeting is a broad band excitation phenomenon containing a high frequency spectrum in the range of 100 to 300 Hz (cycles per second), the control surface cannot be set in high frequency motion. Consequently, its utility is necessarily limited to low frequency modes of vibration. U.S. Pat. No. 5,549,260 to Reed, III discloses an active control device and method for aircraft tail buffet alleviation. The device employs a rotatable slotted cylinder (RSC) partially embedded in the spanwise direction within the contour of the tail surface. Tail tip accelerations were used as the input signals to activate the RSC device to oscillate about +/xe2x88x9215 degrees at required frequencies. In the open position, the upper surface air stream is allowed to pass through the RSC device and exit at the lower surface. Thus, the diversion of airflow must take place at high frequencies for the system to be effective. However, flow continuity can only exist at low frequencies, while at high frequencies the flow separates and the RSC device becomes ineffective. Moreover, the installation of the RSC device weakens the primary structure.
U.S. Pat. No. 5,375,794 to Bleeg discloses an apparatus and method for reducing aircraft loads resulting from atmospheric turbulence and gusts. The primary objective of this disclosure is to improve ride quality and reduce unwanted side loads by commanding a canceling rudder position. However, this device does not attempt to alleviate dynamic loads that result from buffeting.
U.S. Pat. No. 3,734,432 to Low discloses a flutter suppression device. This invention uses a number of pairs of leading and trailing edge control surface pairs in conjunction with a flight control system to suppress the on-set of flutter instability. The major drawback of this approach is that the control surfaces are used by the flight control system to maintain the stability of the aircraft. But, the flutter suppression device commands the same control surfaces. The dual-purpose control algorithms are often undesirable and pilots discourage such devices from safety consideration viewpoint. Besides, if flutter instability of n vibration modes is to be suppressed, this approach requires n/2 pairs of leading and trailing edge control surfaces. These are too many for a flight control system to be reliable and economical.
It was with the knowledge of the foregoing state of the technology that the present invention has been conceived and is now reduced to practice. An active control surface modal (ACSM) device for aircraft buffet and gust load alleviation, and flutter suppression, embodied by this invention is different from all of the devices reviewed above. Furthermore, this system can easily be implemented in existing as well as in new aircraft without significant alterations in the design.
The present invention, then, relates to an Active Control Surface Modal (ACSM) device that generates unsteady aerodynamic damping to alleviate aeroelastic structural instability, vibration and dynamic loads. An active control surface modal deformation is created by means of a pair of antagonistically activated actuators. Since the masses of upper and lower surface skins are lighter, the modal deformations can be activated at high frequencies that can encompass wide band spectrum of buffet, gust and flutter problem areas. An independent closed loop active control system is used to activate the ACSM modes. Measured acceleration sensors are used as the feedback signals to compute the coefficients of the control law that is designed to suppress the dynamic environments such as buffet, gust and flutter. Then a computerized control system algorithm outputs a series of voltage signals that pass through power amplifiers to activate the actuators. Then the ACSM device generates unsteady aerodynamic damping in out-of-phase with the external (buffet/gust) or self excited air loads (flutter) to reduce or eliminate the undesirable dynamic effects on the aircraft. Some buffet analyses of a vertical tail, typical of a fighter aircraft, show 80 per cent reduction in peak stress, which amounts to an eight fold fatigue life extension of the vertical tail. This improvement could save millions of dollars per aircraft in its service life.
A primary feature, then, of the present invention is the provision of a dynamic load alleviation system which successfully suppresses flutter, buffet, and gust loads to which the airfoil is subjected.
Another feature of the present invention is the design, manufacture, and operation of an active control surface modal device embodying the invention.
Still another feature of the present invention is the provision of such a system which provides overwhelming aerodynamic damping in a wideband frequency spectrum so that buffet, gust and flutter associated dynamic problems can be attended to by means a single device.
Yet another feature of the present invention is the provision of such a system provided with an independent active control algorithm so that there is no interference with the flight control system.
Still a further feature of the present invention is the provision of such a system in which the modified control surface preserves all the entities of a conventional control, being able to rotate about the normal hinge line.
Yet a further feature of the present invention is the provision of such a system which can be economically retrofitted to existing aircraft and even more economically be implemented into new aircraft.
Other and further features, advantages, and benefits of the invention will become apparent in the following description taken in conjunction with the following drawings. It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory but are not to be restrictive of the invention. The accompanying drawings which are incorporated in and constitute a part of this invention, illustrate one of the embodiments of the invention, and together with the description, serve to explain the principles of the invention in general terms. Like numerals refer to like parts throughout the disclosure.